1. Field of the Invention
The present invention relates to an optical transmission system and, more particularly, to an optical transmission system having an optical surge suppression function.
2. Description of the Related Art
For medium and long-range optical transmission, an optical amplifier for optically amplifying signal light directly is arranged on a transmission line. As the optical amplifier is widely used an optical fiber amplifier for optically amplifying signal light with an amplifying optical fiber containing a rare earth element.
The amplifying optical fiber containing a rare earth element is connected to a transmission line to receive signal light input thereinto. Pumping light output from a pumping light source is input into the amplifying optical fiber through an optical wavelength division multiplexer (WDM).
Part of the signal light amplified by the amplifying optical fiber is branched by an optical branching device. The branched light is received by a detector to detect the output level of the light. The output of the pumping light is feed-back controlled by the control of an injection current into the pumping light source by a pumping light control circuit in accordance with the detected output level of the light.
The optical amplifier is used in an optical transmission system having operable transmission lines and auxiliary transmission lines, for example, an optical add drop multiplexer ring system (OADM ring system) to which a wavelength multiplex optical transmission is applied, and an optical cross connect transmission system. The optical ADM ring system has auxiliary transmission lines in addition to operable transmission lines in order to ensure the reliability of the optical transmission system. When a failure occurs in part of the system or a transmission line, an optical switch arranged at a node is used to switch between a operable transmission line and an auxiliary transmission line.
In the optical ADM ring transmission system and the optical cross connect transmission system, at the time of switching between the operable transmission line and the auxiliary transmission line at each node, signal light is suddenly input into the optical amplifier arranged on the switched transmission line from an optical input break state.
This sudden input of the signal light may occur also when the transmission path is switched in the optical cross connect transmission system.
When the signal light is suddenly input, the feed-back control of the optical amplifier for the adjustment of the output level of light from the pumping light source cannot follow the sudden rise with the result that the optical amplifier generates an optical surge as an output. The optical surge causes a failure in an optical receiving unit of a node for receiving because the optical surge is further increased by multi-stage linear relay. A definite countermeasure against this optical surge is not taken at the moment.
It is an object of the present invention to prevent an optical surge in an optical direct amplifier arranged on a switched transmission line at the time of switching between operable/auxiliary transmission lines at each node or switching the transmission path in an optical ADM ring transmission system or optical cross connect transmission system.
The optical transmission system having an optical surge suppression function of the present invention comprises a dummy light source for inputting dummy light different from signal light into an optical amplifier in addition to optical amplifiers for optically amplifying input signal light directly. The optical amplifier in the above optical transmission system comprises a rare earth element doped optical fiber for amplifying input signal light to output the amplified signal light, a pumping light source for outputting pumping light, a first optical wavelength multiplexer for inputting the pumping light into the rare earth element doped optical fiber, an optical branching device for branching part of the amplified signal light to output the branched signal light, and a pumping light control unit for controlling the output of pumping light based on the output level of the branched light.
The dummy light source comprises a dummy light output source for outputting dummy light and a second optical wavelength multiplexer for inputting the dummy light into the rare earth element doped optical fiber.
Further, in the optical transmission system having an optical surge suppression function of the present invention, the dummy light contains a self-oscillating clock. The dummy light output source comprises a signal light identifying unit for identifying whether the input light is dummy light or not and a clock control circuit for including a self-oscillating clock in the dummy light output from the dummy light source when the input light is dummy light and not including the self-oscillating clock in the dummy light output from the dummy light source when the input light is not dummy light.
The optical transmission system having an optical surge suppression function of the present invention comprises optical transmission lines including operable transmission lines and auxiliary transmission lines, ADM nodes, a regenerative repeating node and optical amplifiers, arranged on the operable transmission lines and the auxiliary transmission lines, for optically amplifying signal light. Each of the ADM nodes comprises an optical depultiplexing/inserting unit for separating specific signal light from signal light transmitted through the operable transmission lines or the auxiliary transmission lines and inserting the specific signal light and a transmission line switching unit for switching between the operable transmission lines and the auxiliary transmission lines. The regenerative repeating node comprises optical receiving units for receiving signal light transmitted through the operable transmission lines or the auxiliary transmission lines and optical transmitting units for transmitting signal light by regenerative repeating. In the optical transmission system of the present invention, the regenerative repeating unit further comprises a dummy light output unit for transmitting dummy light to the optical transmitting units.
The above regenerative repeating unit of the optical transmission system of the present invention transmits a transmission line clock to an optical transmitting unit connected to a transmission line in operation out of the operable transmission lines and the auxiliary transmission lines. On the other hand, the regenerative repeating unit further comprises the self-oscillating clock sources for synchronizing an optical transmitting unit connected to a transmission line out of operation with a self-oscillating clock to transmit light.
The regenerative repeating node comprises an input break identifying unit and a clock control unit. The input break identifying unit identifies an input break of signal light input from the operable transmission lines and the auxiliary transmission lines. The clock control unit controls the switching of a clock so that input light is synchronized with a transmission line clock extracted from main signal light when it identifies that the input light is main signal light and with a self-oscillating clock source when it identifies that the input light is dummy light.
The optical transmission system having an optical surge suppression function of the present invention is an optical transmission system having the above optical surge suppression function and comprises optical transmission lines including two or more operable transmission lines and auxiliary transmission lines and a center node for transmitting a plurality of signal light having different wavelengths to the respective operable transmission lines or auxiliary transmission lines and receiving signal light from the respective operable transmission lines or auxiliary transmission lines.
The optical demultiplexing/inserting unit comprises a wavelength demultiplexinq unit for extracting and demultiplexing signal light having an arbitral wavelength and a wavelength multiplexing unit for multiplexing the wavelength of signal light transmitted from the ADM node. The dummy light output unit of the present invention outputs the dummy light as a false signal or a continuous light having a wavelength different from the wavelength of the transmission line.
The optical transmission system having an optical surge suppression function of the present invention can be constituted such that a center node, ADM nodes and a regenerative repeating node are interconnected by optical transmission lines in the configuration of a ring. As a result, the present invention can be applied to an optical ADM ring optical transmission system.
Meanwhile, the optical transmission system having an optical surge suppression function of the present invention can be applied to an optical cross connect system that will be described hereinafter as another embodiment of the present invention. This optical transmission system comprises WDM terminal equipment, a plurality of nodes and optical amplifiers arranged between the respective nodes. The WDM terminal equipment includes optical transmitting units for transmitting a plurality of signal light having different wavelengths and optical receiving units for receiving a plurality of signal light having different wavelengths. The plurality of nodes are connected to the WDM terminal equipment through a plurality of transmission lines and switch the path of signal light having a arbitrary wavelength out of the wavelengths of signal light input from the input ports. Each node comprises a dummy light output unit for outputting the dummy light to transmit the dummy light to a transmission line.
The node comprises a wavelength demultiplexing unit for demultiplexing signal light having a specific wavelength out of the wavelengths of input signal light and a regenerative repeating unit for carrying out the regenerative repeating of the signal light demultiplexed by the wavelength demultiplexing unit.
The dummy light is transmitted to a transmission line between unused nodes at the regenerative repeating unit connected to an unused input port.
The dummy light output unit outputs dummy light as a false signal or a continuous light having a wavelength different from the wavelength of the transmission line clock. In the optical ADM ring optical transmission system, light is always transmitted to the operable/auxiliary transmission lines by causing an optical transmitting unit connected to a transmission line out of operation to carry out transmission in synchronism with a self-oscillating clock. Therefore, it is possible to suppress the occurrence of an optical surge without a break of input into an optical direct amplifier arranged on a path between adjacent nodes.
In the optical cross connect system, dummy light is transmitted to a path between unused nodes in synchronism with a self-oscillating clock of the regenerative repeating unit of each node, thereby making it possible to suppress the occurrence of an optical surge without a break of input into an optical direct amplifier arranged on a path between adjacent nodes.